How do I use the cycles Denoiser?
The denoiser is a wonderful part of the cycles render process. It runs after the tile has rendered, but during the render process. It takes information about the scene and calculations cycles makes to turn a noisy image into a remarkably smooth render.
Because it is part of the render process it works better then any filter you could throw on the image after the fact.

This image was render at 10 samples, with the denoiser on at default settings. Notice even the shadows are smooth.


Now here is the exact same render, but without the denoiser. (click for full size to see the difference)




The denoiser is at the bottom of the render layers tab in the properties window. Simply check the checkbox and open up the Denoising section.



Now given that you are not using Progressive Refine, and have blender 2.79 or higher then you are all set.

The three main settings are Radius, Strength, and Feature Strength.


Radius is how far the denoiser looks around each pixel.
Strength think of this as the strength of the over all denoising filter. Lower numbers are sharp (noisy) image, a value of one is full denoising (can kill small details).
Feature Strength controls the strength of a pre-filter, before the denoiser works its magic. Here again low values equal more detail, and a value of one is full strength.


The manual page for the denoiser. The release notes from 2.79.

Note that Denoising will use a lot more resources from your computer and might even yield out of memory errors. Denoising needs the information of all of the adjacent rendered tiles to work, meaning that for every tile being rendered blender will keep in RAM the information of all the surrounding tiles and will only release that ram once a tile has been denoised.